Where would you expect to find the 1H NMR signal of (CH3) 2Mg relative to the TMS signal? (Hint: Magnesium is less electronegative than silicon.)

The methyl protons of (CH3) 2Mg are in a more electron rich environment than the methyl protons of TMS. Thus the protons of (CH3) 2Mg show a signal upfield from TMS.

Explanation:

Mg is less electronegative than Si. The methyl protons of (CH3) 2Mg are in a more electron-rich environment than the methyl protons of TMS. This electron density shields the protons of (CH3) 2Mg from the applied magnetic field. Therefore, they sense a smaller effective magnetic field than TMS. The effective magnetic field is directly proportional to the frequency. Thus the protons of (CH3) 2Mg show a signal at a lower frequency than TMS. In other words, protons of (CH3) 2Mg show signal upfield from TMS.